Single mode optical fibres are designed to support only a single propagation mode per polarization direction for a given wavelength. Single mode optical fibres usually have a relatively small core diameter (e.g., a diameter of only a few micrometers) and a small refractive index difference between the core and cladding. The mode radius is typically a few microns. Mode field diameter is one of the unique characteristic of single mode fibres as is cut-off wavelength. The cut-off wavelength is the wavelength above which the optical fibre will allow only single mode transmission (multiple modes cease propagation), and below the cut-off wavelength multimode transmission starts.
Fibre-to-fibre coupling using collimating lenses through a free space component over a large spectral range (broadband) is fundamentally difficult because of the change in the mode field diameter (MFD) of the fibre with wavelength, and hence its corresponding change in output/acceptance angle (numerical aperture (NA)), which can change the collimating profile of light exiting the collimating lens thus compromising coupling efficiency. There are believed to be no known solutions to this problem.